1. Field of the Invention
The present invention relates to a non-contact type pickup coil. More particularly, the present invention relates to a pickup coil having adjustable characteristic frequency for accurately detecting number of revolutions and rotational position of engines.
2. Description of the Background Art
There are known pickup coils which are widely used as sensors for detecting the number of revolutions and the rotational position of engines in a non-contact manner. Such known pickup coils are generally mounted at a position near the engine, on a structural component which is subjected to an influence of engine vibrations. When a pickup coil is significantly vibrated, such as due to engine vibrations, detection signals might get noisy or distorted, which lowers detection accuracy of the pickup coil.
The Japanese Patent document JP-A-2006-266109 discloses a technology to integrate a center-of-gravity-position adjusting member in a pickup coil for adjusting the position of the center of gravity of the pickup coil in order to dampen vibrations of the pickup coil amplified by displacement between the position of the center of gravity of the pickup coil and the fixed position of a mounting object.
The pickup coil vibrates significantly, not only when the position of the center of gravity thereof and the fixed position of the mounting object do not match, but also when the engine vibrations match the characteristic frequency of the pickup coil. Hence, when this happens, the pickup coil resonates. However, in the related art described above, the problem of resonance of the pickup coil is caused when the engine vibrations and the characteristic frequency of the pickup coil match is not taken into consideration.
The present invention has been made to overcome such drawbacks of the existing pickup coils. Accordingly, it is one of the objects of the present invention to solve the above-described problem in the related art, and to provide a pickup coil having a resonant frequency which is easy to adjust to a desired characteristic frequency.